Delay locked loop with selectable delay

ABSTRACT

A DLL includes a control module coupled with a phase detect signal. The phase detect signal is used by a control module to generate feedback and output select signals. The feedback and output select signals are each coupled to a multiplexer. Each multiplexer is coupled to a Multi-Tap Delay Line (MTDL). The MTDL provides a plurality of delayed signals that are selectable by the two multiplexers. The first multiplexer, coupled to the feedback select signal, selects a feedback clock signal. The second multiplexer, coupled to the output select signal, select a DLL output signal. The control module may receive other signals, such as a delay select signal, that may be used to program or set the delay of the output signal. In addition, a plurality of output signals may be available from the DLL.

GOVERNMENT RIGHTS

The United States Government has acquired certain rights in thisinvention pursuant to Contract No. DTRA01-03-D-00018 and Delivery No.DTRA01-03-D-0018-0003 awarded by the Defense Threat Reduction Agency.

FIELD

The present invention relates generally to a delay locked loop, and moreparticularly, a delay locked loop with a selectable delay.

BACKGROUND

Synchronous systems often employ Phase Locked Loops (PLLs) and DelayLocked Loops (DLLs) in order to synchronize an internal clock signal toan external clock signal. PLLs, as the name implies, synchronize anoutput signal with an external signal. A PLL receives the externalsignal as an input signal and generates the output signal. The outputsignal is generated by a combination of elements that include a chargepump and a voltage controlled oscillator. The output signal, however, isnot the same signal as the input signal. The output signal is agenerated signal that is designed to have the same frequency (ormultiple of the frequency) of the input signal.

DLLs, on the other hand, synchronize an output signal, by delaying aninput signal. The output signal is a delayed version of the inputsignal. The input signal is delayed so that the output signal is at adesired phase. For example, an Application Specific Integrated Circuit(ASIC) may use a DLL so that it can synchronize an incoming data streamfrom a processor with the processor's associated clock signal. Oftentimes, the data stream and the clock signal may not be received by theASIC at the same time. The received clock, for example, may be afraction of a period ahead of the received data stream (due totransmission delays). In order to process the data stream correctly, theASIC may require that the received data stream and the clock be inphase. The ASIC may employ a DLL to delay the received clock signal sothat it is delayed to be in phase with the data stream. However, inother ASIC designs, a phase difference, such as 90 degrees, may bedesired. Circuits down stream may use such a phase difference to retimereceived data, for example.

One limitation to conventional DLLs is that they may offer only alimited number of available delay times or phases. For example, phasedelay increments of a quarter phase may only be available to a givenDLL. In addition, in conventional DLLs, increasing the number of delayincrements may increase the overhead associated with the DLL (e.g.,size, power consumption, etc.).

Conventional DLLs are also limited because they are not easilyprogrammable. A circuit employing a DLL may have requirements for thephase, or delay of a signal that is application specific. For example,in the ASIC described above, the ASIC may need to be reprogrammed for adifferent application, the delay requirements of the ASIC may change, ora new DLL may need to be fabricated with a specific delay time.

Therefore, there is a need for an improved DLL.

SUMMARY

A delay locked loop (DLL) and a method of operation are presented.

In one example, a DLL includes a control module coupled with a phasedetect signal. The control module uses the phase detect signal togenerate feedback and output select signals. The feedback and outputselect signals are coupled with two multiplexers coupled with aMulti-Tap Delay Line (MTDL). The MTDL provides a plurality of delayedsignals that are selectable by the two multiplexers. The delayed signalsmay represent different phases of a reference clock signal. The firstmultiplexer uses the feedback select signal to choose one delayed signalas a feedback clock signal. The second multiplexer uses the outputselect signal to choose one delayed signal as the DLL output signal.

In another example, the DLL is coupled with a π-phase detect signal, inorder to prevent the DLL from locking onto the π-phase (or an oddmultiple of the π-phase) of the feedback clock signal. The DLL may alsoproduce a lock detect signal that may be used by internal and externalcircuits to the DLL to determine the accuracy of the output signal.

In a further example, the control module is programmable so that a usermay set the delay or phase of the output signal. A delay select signalmay be used to program the control module.

In even further examples, the phase detect, feedback select, and outputselect signals may include more than one component. These components maybe phase components such as zero-phase and 2π-phase components. Thezero-phase and 2π-phase components may be used by a logic circuit withinthe DLL to perform mathematical operations. These mathematicaloperations may provide any number of calculable output signal phases. Inaddition, inverters may be added to one or more output signals of theDLL to increase the number of available output signals having differentphases.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain examples are described below in conjunction with the appendeddrawing figures, wherein like reference numerals refer to like elementsin the various figures, and wherein:

FIG. 1 a is block diagram of a Delay Locked Loop (DLL);

FIG. 1 b is a block diagram of another DLL;

FIG. 2 a is a circuit diagram of a phase detector;

FIG. 2 b is a circuit diagram of a circuit used to generate π-phasedetect signals;

FIG. 3 a is a block diagram of a Multi-Tap Delay Line (MTDL);

FIG. 3 b is a circuit diagram of a delay cell;

FIG. 4 is a circuit diagram of a multiplexer;

FIG. 5 a is a block diagram of control module;

FIG. 5 b is a circuit diagram of a clock distribution buffer, azero-phase module, and 2π-phase module;

FIG. 5 c is a circuit diagram of a feedback and output select module;and

FIG. 6 is a block diagram of a method of calculating a phase of anoutput signal.

DETAILED DESCRIPTION

A delay locked loop (DLL) with a selectable delay is presented. The DLLmay delay an input reference clock signal in a variety of methods. Thedelayed reference clock signal may be delayed so that a desired phase isachieved as an output signal of the DLL. In one example of delaying thereference clock signal, the DLL includes a control module that, inoperation, uses a received phase detect signal to derive feedback andoutput select signals. The feedback and output select signals are usedby two separate multiplexers (MUXes) coupled with a Multi-Tap Delay Line(MTDL) in order to select an appropriate feedback clock signal andoutput signal. The feedback clock signal is used by a phase detector todetect the phase difference between the feedback clock signal and thereference clock signal. The MTDL receives a reference clock signal andprovides a plurality of delayed signals to both MUXes so that the MTDLmay be “tapped” and thereby provide both the feedback clock signal andthe output signal. The output signal is a delayed reference clocksignal.

In a second example, the DLL may delay a reference signal by determininga delay time by using an input delay select signal. The delay selectsignal may also be coupled with the control module so that the outputselect signal output from the control module reflects a desired delay ofthe output signal. By using the delay select signal, the control moduleand thus the DLL is programmable for outputting a reference clock signalwith a desired delay or phase.

In a third example, the select, offset, and feedback signals maycomprise more than one component representing various phases of thereference signal. Using these different components, mathematicaloperations may be performed by a logic circuit in order to provide aplurality of output signals having different delays or phases.

Turning now to FIG. 1 a, a DLL 9 receives a reference clock signal(REFCLK) and a feedback clock signal (FBKCLK) on respective signal lines20 and 22. Output from the DLL 9 is an output signal (OUTPUT0) on signalline 23. Within DLL 9, REFCLK and FBKCLK are communicated to a phasedetector 50. Phase detector 50 produces a phase detect signal (PHASEDET)on signal lines 24 a and 24 b. Signal lines 24 a and 24 b carryrespective up and down components of PHASEDET.

Control module 200 is coupled to receive PHASEDET and produces afeedback select signal (FBKSEL) on signal line 25 and an output select(OUTSEL) signal on signal line 26. Feedback MUX150 a, coupled with MTDL100, receives FBKSEL and produces FBKCLK. Output MUX 150 b, also coupledwith MTDL 100, receives OUTSEL and produces OUTPUT0.

MTDL 100 is coupled with REFCLK and provides a plurality of delayedsignals (i.e., delayed REFCLKs) to MUXes 150 a and 150 b. Signal lines27 a and 27 b carry delayed REFCLK signals. Many delayed REFCLK signalsare available on signal lines 27 a and 27 b. Each delayed REFCLK signalis coupled to MUXes 150 a and 150 b and may be selectable by MUXes 150 aand 150 b.

In operation, DLL 9 employs phase detector 50 in order to compare REFCLKand FBKCLK and determine if the two signals are in phase. Phase detector50 output, PHASEDET, is a signal indication of whether REFCLK and FBKCLKare in phase, or whether REFCLK is ahead (leads) or behind (lags)FBKCLK. As described above, PHASEDET may comprise up and downcomponents, as is shown in FIG. 1 a. The up component may indicate thatDLL 9 should increase the delay of FBKCLK and the down component mayindicate that DLL 9 should decrease the delay FBKCLK.

In response to receiving PHASEDET, the control module 200 generatesFBKSEL and OUTSEL so that MUXes 150 a and 150 b select an appropriatedelayed REFCLK for FBKCLK and OUTPUT0. MUXes 150 a and 150 b may beconventional multiplexers, or some other type of logic circuitry used toselect an appropriate signal from an array of available signals. Inaddition, FBKCLK and OUTPUT0, selected from MUXes 150 a and 150 b, arenot necessarily the same. As DLL 9 monitors REFCLK, FBKCLK should bedelayed so that it is in phase with REFCLK. OUTPUT0, however, may be adelayed representation of REFCLK.

The delay of OUTPUT0, in relation to REFCLK, may be applicationspecific, or programmed into the control module 200. A delay selectsignal (DELAYSEL) may be used to program the control module so that itincreases or decreases the value of OUTSEL. DELAYSEL may represent aprogrammed time by a user. MUX 150 b will respectively select a longeror shorter delay time for REFCLK based on the programmed time. Thisprogrammable configuration offers a second way to delay REFCLK. DELAYSELis described in further detail with respect to FIG. 1 b.

Creating a delayed OUTPUT0 (i.e., a REFCLK that is at a different phasethan its input) may be desirable as some applications may require aspecific delay time for an input REFCLK. For example, a data processingcircuit may use the DLL 9 so as to synchronize two different streams ofincoming data. Both data streams may have the same frequency but areoffset from each other by a known phase difference. The DLL 9 could beprogrammed and subsequently used to offset one of the data streams sothat both of them are in sync with each other or at a desired phasedifference.

As described above, the MTDL 100 may provide a plurality of delayedsignals. Each delayed signal within the plurality of delayed signals isa delayed REFCLK. Some configuration may only use some of the outputsignals available from MTDL 100. If resolution is not as important forFBKCLK, signal line 27 a, for example, may represent a 5-bit number.Signal line 27 b, however, may represent a 7-bit number. Signals lines27 a and 27 b may represent larger or smaller bit numbers.

The maximum resolution of the delay of the DLL 9 is set by the number ofavailable delayed signals on signal lines 27 a and 27 b. For example, ifMTDL 100 has 60 taps, 60 phases may be available. The 60 taps may beused to span a phase delay range of 0 to 3π. Each tap in the MTDL 100would then represent a phase delay of 9 degrees or π/20. By increasingthe number of taps, the resolution can be increased or the span of thephase delay range may be increased. On the other hand, by decreasing thenumber of taps, the resolution may be subsequently decreased.

Alternative to the example DLL 9 in FIG. 1 a, DLL 10 in FIG. 1 b mayalso be used to delay REFCLK. The DLL 10 in this example maintains allof the components of DLL 9 (i.e., MUXes 150 a and 150 b, MTDL 100,control module 200); however, additional components to some of thesignals are added. Also added are the signals π-detect (PIDETECT) onsignal lines 30 and 31 and DELAYSEL on signal line 32. DLL 10 outputs aplurality of output signals (OUTPUT0, OUTPUTPI/2, OUTPUTPI,OUTPUT3PI/4), on signal lines 23 a-d. The plurality of output signalsprovides a third way of delaying REFCLK. A user may select OUTPUTPI/2 asan output. OUTPUTPI/2 has a π/2 phase difference in comparison toREFCLK. In addition to the plurality of output signals, DLL 10 alsooutputs a lock detect signal (LOCKDET) on signal line 34.

Control module 200, in this example receives PHASEDET on signal lines 24a-d. Signal lines 24 a-d carry several components of PHASEDET. Signallines 24 a and 24 c carry up pulses and signal lines 24 b and 24 d carrydown pulses. In addition, signal lines 24 a and 24 b carry a zero-phasecomponent of PHASEDET and signal lines 24 c and 24 d carry a 2π-phasecomponents PHASEDET. The zero-phase and 2π-phase components of PHASEDET,as well as other signals, are made available to the control module 200so that the control module 200 may derive a desired phase component ofOUTSEL. Deriving a desired phase component of OUTSEL is described withreference to FIG. 6.

The up and down components as well as the zero phase and 2π-phasecomponents of PHASEDET may be viewed as distinct signals. In thisrespect, PHASEDET may be viewed as either a single phase detect signalor four distinct phase detect signals. PHASEDET may be a singlemulti-bit signal where a bit or several bits represent a component suchas an up or down component. Alternatively, four distinct single-bit ormulti-bit signals may make up PHASEDET. In even further examples,PHASEDET may comprise additional components to further characterize thedelay, or phase, of REFCLK. Such characterization could include π/2 orπ/4 phase components of PHASEDET.

In order to generate PHASEDET, phase detector 50 may comprise a varietyof logic circuits to evaluate REFLCK and FBKCLK. In FIG. 2 a, two logiccircuits are used to create the phase detector 50. In this example, Dflip-flops 52 and 54 are coupled with FBKCLK and REFCLK on respectivesignal lines 20 and 22 a. Both of these flip-flops output the zero-phaseup and down components of PHASEDET on signal lines 24 a and 24 b. Dflip-flop 52 tracks REFCLK and FBKCLK and evaluates whether REFCLK isahead of FBKCLK. D flip-flop 52 goes “high” to indicate that REFLCK isahead of FBKCLK. In contrast, D flip-flop 54 tracks REFCLK and FBKCLKand it goes “high” when FBKCLK is ahead of REFCLK. The outputs of Dflip-flops 52 and 54 may be coupled with buffers so as to adjustPHASEDET on signal lines 24 a and 24 b.

D flip-flops 56 and 58, on the other hand, output the 2π-phase up anddown components of PHASEDET on signal lines 24 c and 24 d. D flip-flops56 and 58 are coupled with FBKCLK and REFCLK on signal lines 20 and 22 brespectively. When D flip-flop 56 goes “high”, REFCLK is ahead ofFBKCLK. When D flip-flop 58 goes “high”, FBKCLK is ahead of REFCLK.

As mentioned above, many different configurations of logic circuits maybe arranged in order to create a phase detector 50. A radiation hardenedphase detector 50 may be created by adding redundant D flip-flops to theexample phase detector 50 in FIG. 2 a. These additional D flip-flops maybe coupled with D flip-flops 52-58 so that they are in a voterconfiguration. A voter configuration of D flip-flops 52-58 would onlyallow up and down components of the zero and 2π phase components of thephase signal to be generated when the majority of redundant D flip-flops“agree” on the correct output signal. If a radiation event, such as aSingle Event Upset (SEU), were to occur at one of these redundantflip-flops, an erroneous phase signal could be prevented by not allowingthe phase signal to transition until all of the redundant D flip-flopsare in agreement.

Returning to FIG. 1 b, in addition to receiving PHASEDET, control module200 is also coupled to receive PIDETECT and DELAYSEL. PIDETECT allowsthe control module 200, and the DLL 10, to detect when FBKCLK is lockedonto the π-phase, or an odd multiple of the π-phase of REFCLK. UtilizingPIDETECT prevents the control module 200 from locking onto REFCLK at itsπ-phase. Without PIDETECT, the DLL 10 may not send up or down componentsof PHASEDET when it is locked onto the π-phase of REFLCK. In the exampleof FIG. 1 b, PIDETECT has two components, a zero-phase component onsignal line 30 and a 2π-phase component on signal line 31.

A circuit 60 for generating PIDETECT is illustrated in FIG. 2 b. Two XORlogic gates 62 and 64 are coupled with FBKCLK and RFCLK. XOR gate 62 iscoupled with signal lines 20 and 22 d and produces the zero-phasecomponent of PIDETECT on signal line 30. XOR gate 64 is coupled withsignal lines 20 and 22 b and produces the 2π-phase component of PIDETECTon signal line 31. In operation, when REFCLK and FBKCLK are out ofphase, the zero-phase and 2π-phase component of PIDETECT on respectivesignal lines 30 and 31 go “high”. Circuit 60 may be included in thephase detector 50 or in another component of the DLL 10, such as thecontrol module 200. In some examples, PIDETECT may have a margin ofphase tolerance. That is, when FBKCLK is in the region of, or a fewdegrees out of the π-phase of REFCLK, it still may, nevertheless,indicate that it is locked onto the π-phase.

Returning again to FIG. 1 b, DELAYSEL, as described above, is used todetermine the phase offset, or delay of OUTSEL. DELAYSEL may be amulti-bit signal that is added to FBKSEL so as to produce OUTSEL. Inother examples, DELAYSEL could be sent from other circuitry, orprogrammable by way of a dip switch. A user could input the DELAYSEL bysetting the dip pins on the dip switch for the appropriate delay. Manyother types of software, or hardware could be implemented so as toprovide DELAYSEL.

Also shown as output from the control module 200 is LOCKDET on signalline 34. LOCKDET is coupled with internal circuitry within DLL 10 and/orexternal circuitry to provide status information as to whether or notOUTPUT0 is locked onto REFCLK. If a lock is detected, the OUTPUT0 (aswell as OUTPUTPI/2, OUTPUTPI, and OUTPUT3PI/4) may be viewed as accurateand that OUTPUT0 is at a correct or desired phase. In alternativeexamples, the LOCKDET may be output from other modules within the DLL10, such as MUX 150 b.

In FIG. 1 b, signal lines 22 a and 22 b respectively carry thezero-phase and 2π-phase components of the FBKSEL signal. Signal lines 22a and 22 b respectively carry the zero and 2π-phase components ofFBKCLK. Signal lines 26 a and 26 b respectively carry the zero andπ/2-phase components of OUTSEL. Signal lines 27 a and 27 b carryzero-phase components of delayed REFCLKs output from the MTDL 100.Signal lines 27 c and 27 d respectively carry the 2π-phase and π/2-phasecomponents of delayed REFCLKs.

The illustrated example of FIG. 1 b may be reconfigured to includevarious combinations of the signals within DLL 9 or 10 in order toachieve a desired output. The illustrated examples in FIGS. 1 a and 1 bshould not be viewed as limited to only those examples. Many of thesignal paths may also be combined with delay buffers or inverters tooptimize OUTPUT0, OUTPUTPI/2, OUTPUTPI, and OUTPUT3PI/4.

The MTDL 100 and MUXes 150 a and 150 b use the respective zero and2π-phase components of OUTSEL and the zero and π/2-phase components ofOUTSEL in order to generate FBKCLK, OUTPUT0, and OUTPUTPI/2. An exampleMTDL 100 is illustrated in FIG. 3 a. The MTDL 100 includes a series ofdelay cells 105 a-p. REFCLK is input into the MTDL 100 and taps 150 a-poutput delayed REFCLKs. Each delay cell has an input and at least oneoutput. The delay cells 105 a-p are each coupled to receive a clocksignal. For example, delay cell 105 a is coupled to receive REFCLK.Delay cell 105 b, on the other hand, is coupled to receive an outputfrom delay cell 105 a. Delay cells 105 a-p provide as output therespective taps 110 a-110 p. Taps 10 a-d are coupled with MUXes 150 aand 150 b via signal lines 27 a-d.

In operation, when the MTDL 100 receives REFCLK, the signal is delayedby delay cell 105 a. The amount of delay is determinable by the designof the delay cells within MTDL 100. The delayed REFCLK is available attap 10 a and it is also sent to delay cell 105 b. In the same manner,delay cell 105 b delays the delayed signal even further. The furtherdelayed signal is available at tap 110 b and it is sent to delay cell105 c. A plurality of delay signals becomes available by adding delaycells. In the example of FIG. 3 a, sixteen delay cells are shown.Depending on the application, however, a larger or smaller number ofdelay cells may be implemented. For example, a larger number of delaycells may be used to provide available phase delays in the range of zeroto 3π.

In FIG. 3 a, REFLCK is shown coupled to the MTDL 100; in otherconfigurations, however, other input clocks signals may be used. TheMTDL 100 is not limited to only receiving the REFCLK. Also, variousbuffers or delay elements may be used to couple the REFCLK or otherinput clock signals with the MTDL 100.

An example delay cell 105 a is illustrated in FIG. 3 b. Delay cell 105 acomprises buffer delays 112, 114 and capacitor 116. In some examples,capacitor 116 may be an intrinsic parasitic capacitance or a discretecomponent, for example. Buffer delay 112 has an output coupled withinputs of buffer delay 114, capacitor 116, and output 118. Buffer delay114 has an output coupled with tap 10 a. Capacitor 116 is coupled withVss.

An input clock signal (i.e., REFCLK) is input at input 120. Buffer delay112 delays the input clock signal and outputs the signal to buffer delay114. The delayed clock signal is output at tap 110 a. The delayed signalfrom buffer delay 112 is also sent to output 118, so that it may befurther delayed by a subsequent delay cell.

Many other types of delay cells may be used in the MTDL 100. A delaycell with a different configuration of delay buffers or capacitors couldbe implemented. Alternatively, different circuit elements, such as thosefound in conventional digital circuits could be used to delay REFCLK.

Once the delayed REFCLKs have been derived, MUXes 150 a and 150 b arethen used to select delayed REFCLKs. An example MUX 150 a, 150 b isillustrated in FIG. 4. MUX 150 a, 150 b is coupled to receive MTDL 100output signals 110 a-p (i.e., via signal lines 27 a-d). In addition, MUX150 a, 150 b also receives input signals 152 and 154 which aredistributed into binary components 152 a-d and 154 a-d respectively.Output from MUX 150 a, 150 b are outputs 156 and 158. A first inputsignal on signal line 152 is used to select one phase component of thedelayed REFCLK (i.e., zero-phase) at output 156. A second input signalon signal line 154 is used to select the phase component (i.e., π/2 or2π) of the delayed REFCLK at output 158.

MUxes 150 a, 150 b include 4 arrays of 2×1 MUXes, 160-166. Each 2×1 MUXreceives two input signals and a MUX select signal. The MUX selectsignal is used to select one of the two input signals as an outputsignal.

The first array of MUXes 160 is coupled to receive MTDL 100 outputs 110a-p as inputs. The least significant binary digit of the signal lines152 and 154, (i.e., 152 d, 154 d) is coupled to the MUX select inputs ofthe first array 160. For example, a first pair of 2×1 MUXes 168 eachreceive MTDL 100 outputs 110 a and 110 b as input. The signal at input152 d is used by the first 2×1 MUX in the pair to select a first outputfor the second array of MUXes 162. Signal 154 d is used by the second2×1 MUX in the pair to select a second output for the second array ofMUXes 162.

In the example of FIG. 4, for output MUX 150 b, input 152 may be used asthe zero-phase component of OUTSEL. Input 154 may then be used as theπ/2-phase component OUTSEL. Alternatively, for feedback MUX 150 a, input152 may be used as the zero-phase component of FBKSEL. Input 154 wouldthen be used as the 2π-phase component of FBKSEL.

The second array of MUXes 162 selects, via inputs 152 c and 154 c,outputs sent from the first array of MUXes 160. The third array of MUXes164, in turn, selects, via inputs 152 b and 154 b, outputs sent from thesecond array of MUXes 162. The fourth array of MUXes 166 ultimatelyselects the output signal available at output 156 and 158 of MUX 150 a,150 b.

As described above MUX 150 a, 150 b may be coupled to FBKSEL or OUTSELat inputs 152 and 154. In order to illustrate how FBKSEL and OUTSEL aregenerated, a block diagram of the control module 200 is illustrated inFIG. 5 a. The control module 200 comprises a clock distribution/buffermodule 210, zero-phase and 2%-phase modules 220 and 230, lock detect240, and feedback and output select module 250. All of the modules inthis example are coupled with signal line 36, which provides a resetsignal to all the modules. Signal line 36, although not shown in FIGS. 1a and 1 b, may provide reset functionality via a reset signal (RESET) toeither DLL 9 or 10.

The clock distribution/buffer 210 is coupled to receive REFCLK. Withinclock distribution/buffer 210, is a series of interconnected Dflip-flops, illustrated in FIG. 5 b. The D flip-flops within the clockdistribution/buffer 210 serve to divide a received clock signal by two.The D flip-flops thereby increase the period of REFCLK. As is shown inthe example clock distribution/buffer 210 of FIG. 5 b, each D flip-flopmay be coupled with buffers or inverters in order to isolate REFCLK intoderived clock signals. The arrangement and configuration of the buffers,inverters, and D flip-flops may depend on the frequency of the signalsand the design parameters of the circuits used in the DLL 10. Outputfrom the clock distribution/buffer 210 are four derived clock signals onsignal lines 212 a-d. More or fewer derived clock signals may beimplemented depending on the configuration of a specific DLL.

Returning to FIG. 5 a, the first derived clock signal on signal line 212a is coupled with the zero-phase and 2π-phase modules 220 and 240. Thesecond derived clock signal on signal line 212 b, zero-phase lock detectsignal on signal line 242 a, PHASEDET on signal lines 24 a and 24 b, andPIDETECT on signal line 30 are coupled with the zero-phase module 220.2π-phase lock detect signal on signal line 242 b, PHASEDET on signallines 24 c and 24 d and PIDETECT on signal line 31 are coupled with the2π-phase module 230.

FIG. 5 b shows a logic diagram of both the zero-phase and 2π-phasemodules 220 and 230. Similar to the clock distribution/buffer 210, thezero-phase module 220 uses D flip-flops and logic gates in order totransform the received up and down components of the zero-phasecomponents of PHASEDET on signal lines 24 a and 24 b into up and downoutput signals at outputs 222 a and 222 b. The logic and D flip-flopsare configured to evaluate the received zero-phase PHASEDET components,the zero-phase lock detect signal on signal line 242 a, along withPIDETECT on signal line 30. This configuration determines if the DLL 9or 10 should send up pulses when PIDETECT indicates that the DLL 9 or 10is locked on a π-phase (or an odd multiple of the π-phase) of REFCLK.For example, the output of the phase detector 50 may indicate that no upor down zero-phase components of PHASEDET are required. However,PIDETECT on signal line 30 may indicate that the DLL 10 is locked on aπ-phase of REFCLK. Signal line 222 a will carry an up signal despite nozero-phase up component of PHASEDET being present. This ultimatelyallows the DLL 10 to continue to cycle through MTDL 100 until theappropriate delayed REFCLK is selected.

Also shown in FIG. 5 b is a logic diagram of the 2π-phase module 230.Similar to the logic diagram of the zero-phase module 220, the 2π-phasemodule 230 uses PIDETECT on signal lines 30 and 31 to determine if anoutput up pulse should be generated. Outputs 232 a and 232 b carryrespective up and down 2π-phase components of PHASEDET. In alternativeexamples, the zero-phase and 2π-phase modules 220 and 230 may compriseadditional logic to evaluate other conditions present in a DLL, such asfast and slow up and down pulses. In this type of implementation, a DLLcould slow down or speed up its response based on the magnitude of theup and down components of the PHASEDET received by the control module200. In even further examples, clock distribution/buffer 210, thezero-phase and 2π-phase modules 220 and 230 may be located in othercomponents of the DLL. The zero-phase and 2π-phase modules 220 and 230may be located in the phase detector 50, for example.

Returning again to FIG. 5 a, signal lines 222 a, 222 b, 232 a, and 232 bare coupled with lock detect module 240 and feedback and select module250. Lock detect 240 determines if the signals received from thezero-phase and 2π-phase modules 220 and 230 are locked. Lock detect 240outputs zero-phase and 2π-phase lock detect signals on signal lines 242a and 242 b. In order to detect a lock condition, lock detect 240 maycomprise circuitry that determines if up and down components of thesignals generated from the zero-phase and 2π-phase modules 220 and 230are changing or are staying in a steady state. The circuitry may alsodetermine if either of modules 220 or 230 has changed states after apredetermined amount of time before generating the zero-phase and2π-phase lock detect signals on signal lines 242 a and 242 b.

Feedback and output select module 250 is coupled with signal lines 242 aand 242 b as well as signal lines 222 a, 222 b, 232 a, and 232 b. Italso receives DELAYSEL on signal line 32 as well as derived clocksignals on signal lines 212 b-d. Depending on the configuration of DLL10, more or fewer signal lines may be coupled with the feedback andselect module 250. Feedback and select module 250 outputs OFFSEL onsignal lines 25 a and 25 b and OUTSEL on signal lines 26 a and 26 b.

A circuit diagram of feedback and select module 250 is illustrated inFIG. 5 c. Feedback and output select module 250 comprises D flip-flops251-256, counters 261 and 266, 2×1 MUXes 271 and 272, adders 281-285,and a shift-right-twice register 286. A logical AND gate 290 may also beused to provide LOCKDET, which is output from feedback and output selectmodule 250.

In operation, the feedback and output select module 250 uses Dflip-flops 251-254 to sample input phase signals from signal lines 222a, 222 b, 232 a, and 232 b from zero-phase and 2π-phase modules 220 and230. The outputs from D flip-flops 251-252 are coupled with counter 261which counts up and down based on the received signals from thezero-phase module 220. Counter 261 is coupled with signal line 212 b inorder to provide a clock signal to the counter. Counter 261 outputs thezero-phase component of FBKSEL on signal line 262. In the example ofFIG. 5 c, FBKSEL is also coupled with D flip-flop 254 andbuffers/inverters in order to sample the zero-phase component of FBKSELat a periodic interval of the derived clock signal on signal line 212 d.A buffered version of the zero-phase component of FBKSEL is also outputat output 25 a. Again, as described above, various configuration ofbuffers and/or flip-flops may be provided to modify a signal so that itis compatible with other circuit components in the DLL 10.

In addition to counter 261 being coupled with D flip-flop 254, counter261 is also coupled with a binary load input 263. Counter 261 may set aninitial count via a signal at load input 264. Counter 261 is alsocoupled with adder 281 in order to create the zero-phase component ofOUTSEL. Essentially, FBKSEL is added to DELAYSEL to create thezero-phase component of OUTSEL at output 26 a.

When the counter 261 receives up and down signals from D flip-flops 251and 252, it increments or decrements the value of FBKSEL. For example, areceived up signal from zero-phase module 220 (via D flip-flop 251), mayindicate that a larger delay should be selected from MTDL 100. When anup signal is received, the counter 261 may increase FBKSEL by “1”.Alternatively, a received down signal may decrease FBKSEL by “1”.

The load input 264 is used to set the initial value of the counter 261.When DLL 10 starts up, it may need to decrease the counter 261 so thatFBKCLK can lock on to REFCLK. By loading an initial value at input 263,the counter 261 can be decremented instead of being pinned at its loweststorable value. If counter 261 is a 5 bit counter, for example, aninitial value of “00011” may be stored at input 263 and it may besubsequently loaded at the startup of DLL 10 by inputting a load signalat load input 264. The zero-phase component of FBKSEL would have thevalue of “3” and it would be used by Feedback MUX 150 a to select thethird delayed REFCLK from MTDL 100 of the zero-phase component ofFBKCLK.

As described above, the zero-phase component of OUTSEL is output atadder 281. DELAYSEL may have a value of “00100”, for example. A signalvalue of “01000” of the zero-phase component of FBKSEL would produce anoutput signal value of “01100” for the zero-phase component of OUTSEL onsignal line 26 a. The zero-phase component of OUTSEL would have thevalue of “12” and it would be used by Feedback MUX 150 b to select thetwelfth delayed REFCLK from MTDL 100 as OUTPUT0.

At initial start-up of the DLL 10, the control module 200 a will firstderive the zero-phase components of FBKSEL and OUTSEL. After thezero-phase components of FBKSEL and OUTSEL are calculated, the 2π-phasecomponent of FBKSEL and the π/2 component of OUTSEL are calculated. Thisis carried out using 2×1 MUXes 271 and 272. MUXes 271 and 272 arecoupled with the zero-phase of the lock detect signal on signal line 242a. Counter 266 will count in the same manner of 261, initially using thesignals sent from D flip-flops 251 and 252. This will pre-set the outputof the counter 266 which outputs the 2π-phase component of FBKSEL onsignal line 267. When the zero-phase component of the lock detect signalon signal line 242 a is detected, however, MUXes 271 and 272 will selectthe 2π-phase up and down components output from D flip-flops 253 and254. When the zero-phase component of the lock detect signal on signalline 242 a indicates “lock”, the counter 266 will continue to count upor down based on up and down signals that it receives. Signal lines 242a (as well as 242 b) may also be coupled with AND gate 290 to produceLOCKDET at output 34. Additionally, in the same manner as counter 278,counter 275 may be loaded with an initial count value.

Similar to D flip-flop 255, D flip-flop 256 may be coupled with the2π-phase component of FBKSEL in order to sample the 2π-phase componentof FBKSEL at a periodic interval of the derived clock signal on signalline 212 d. A buffered version of the 2π-phase component of FBKSEL isalso output at output 25 b. The 2π-phase component of FBKSEL is used byfeedback MUX 150 a to select a delayed REFCLK from MTDL 100 for the2π-phase component of FBKCLK.

In contrast to FBKSEL, OUTSEL has a π/2-phase component instead of2π-phase component. To calculate the π/2-phase component of OUTSELmathematical operations via adders 282-285 and shift-right-twiceregister 286 are performed. The overall equation is as follows:${\phi( \frac{\pi}{2} )} = \frac{( {{\phi( {2\pi} )} + {3{\phi(0)}}} )}{4}$Where $\phi( \frac{\pi}{2} )$is the π/2 phase component of OUTSEL, φ(2π) is the 2π-phase component ofFBKSEL and φ(0) the is zero-phase component of FBKSEL.

Using the mathematical, or algebraic, calculations via adders 282-285and shift-right-twice register 286, the π/2-phase component of OUTSEL isavailable on signal line 26 b. The π/2-phase component of OUTSEL is usedby output MUX 150 b to select a delayed REFCLK from MTDL 100 for theπ/2-phase component of OUTPUTPI/2.

In addition to deriving the π/2-phase component of OUTSEL, any phasecomponent may be calculated for OUTSEL by using a variety of logiccircuit configurations. A method of deriving a phase component of OUTSELis illustrated in the flow diagram of FIG. 6. At block 602, at least twofeedback signals, having different phases, are derived from a referencesignal. This is performed in DLL 10 by deriving the zero-phase and2π-phase components of FBKCLK via phase detector 50, MTDL 100, MUX 150 aand control module 200. Once the signals are derived a logic circuit maybe used to calculate the desired phase of an output signal, as is shownat block 604. This output signal in DLL 10 is the π/2-phase component ofOUTSEL. The logic circuit used to calculate the desired phase in the DLL10 includes adders 282-285 and shift-right-twice register 286.

An example calculation performed by adders 282-285 and shift-right-twiceregister 286 is performed as follows. The zero-phase component of FBKSELon signal line 262 may have a value of “1100”, or 12. Adder 282 has bothof its inputs coupled with signal line 262 and it produces “1000” as anoutput. The output of adder 283 is subsequently added to the zero-phasecomponent of FBKSEL by adder 284 and produces “100100”, or 36, as anoutput. This signal is then added to the 2π-phase component of FBKSEL.If, in this example, the 2π-phase of FBKSEL has a value of “110000”, or48, the output of adder 285 would be “1010100”, or 84. Shift-right-twiceregister 286 then shifts its input signal two times to the right andproduces “10101”, or 21, as an output. DELAYSEL is added to the outputof shift-right-twice register 286 at adder 285.

Many alternative configurations may be used to provide the algebraiccalculation of the π/2-phase component of OUTSEL. In addition, theadders 281-285 and shift-right-twice register 286 may be optimized. Theshift-right-twice register 286, for example, may be optimized to roundup or down so that the least significant bits in the shift-right-twiceoperation are not lost.

Providing the π/2-phase of the select signal allows the DLL 10 toprovide OUTPUTPI/2 and OUTPUT3PI/4 at outputs 23 c and 23 d. Forexample, in FIG. 1 b, OUTPUT0 may be in sync with REFCLK. OUTPUTPI/2,however, may provide a π/2-phase delayed REFCLK signal. Inverters 40 and41, as shown in FIG. 1 b, may be coupled with OUTPUT0 and OUTPUT3PI/4 soas to provide π-phase and 3π/4 phase-delayed REFCLKs.

Overall, the above examples illustrate three ways of selecting a delayof an output signal in a DLL. The selected delay may be used to create adesired phase for a reference clock signal. One example uses the MTDL100 to provide feedback and output MUXes 150 a and 150 b with aplurality of delayed REFCLKs. In another example, DELAYSEL is used todelay the zero-phase and π/2-phase components of OUTSEL. DELAYSELultimately delays the delayed REFCLK selected by output MUX 150 b. Thethird example outputs multiple delayed REFCLKs (i.e., OUTPUT0,OUTPUTPI/2, OUTPUTPI, and OUTPUT3PI/4) at the DLL outputs 23 a-d. In theexample of FIG. 1 b, all three examples are implemented. In the exampleof FIG. 1 a, however, only the example using the MTDL 100 is employed.Alternative examples may use different combinations of theseconfigurations in order to optimize a DLL. Therefore, it should beunderstood that the illustrated examples are examples only and shouldnot be taken as limiting the scope of the present invention. The claimsshould not be read as limited to the described order or elements unlessstated to that effect. Therefore, all examples that come within thescope and spirit of the following claims and equivalents thereto areclaimed as the invention.

1. A Delay Locked Loop (DLL), comprising: a control module coupled toreceive a phase detect signal, the control module deriving a feedbackselect signal and an output select signal based on the phase detectsignal; a Multi-Tap Delay Line (MTDL) coupled to receive a referenceclock signal, the MTDL comprising a plurality of delay taps, each delaytap outputting a delayed reference clock signal, the MTDL, thereby,providing a plurality of delayed reference clock signals; a firstmultiplexer coupled to receive the feedback select signal and theplurality of delayed reference clock signals, the first multiplexerselecting, via the feedback select signal, a first delayed referenceclock signal from the plurality of delayed reference clock signals as afeedback clock signal; and a second multiplexer coupled to receive theoutput select signal and the plurality of delayed reference clocksignals, the second multiplexer selecting, via the output select signal,a second delayed reference clock signal from the plurality of delayedreference clock signals as a first output signal.
 2. The DLL as in claim1, further comprising a phase detector coupled to receive the referenceclock signal and the feedback clock signal, the phase detector derivingthe phase detect signal by measuring a phase difference between thereference clock and feedback clock signals.
 3. The DLL as in claim 2,wherein the phase detector comprises latches coupled together in avoting scheme so as to prevent Single Event Upset (SEU) events.
 4. TheDLL as in claim 1, wherein the DLL is coupled with a π-phase detectsignal, the π-phase detect signal indicating when the feedback clocksignal is lagging the reference clock signal by a phase that is an oddmultiple of π.
 5. The DLL as in claim 1, wherein the DLL produces a lockdetect signal, the lock detect signal indicating that the output signalis at a correct phase.
 6. The DLL as in claim 1, wherein the controlmodule is coupled with a delay select signal, the delay select signalhaving a signal that represents a programmed delay time, and the controlmodule increasing the value of the output select signal by theprogrammed delay time.
 7. The DLL as in claim 1, wherein the phasedetect, feedback select, and output select signals comprise at least twophase components.
 8. The DLL as in claim 7, wherein the phase detect andfeedback select signals comprise zero-phase and 2π-phase components, andthe output select signal comprises zero-phase and π/2-phase components.9. The DLL as in claim 8, wherein the control module comprises a logiccircuit, the logic circuit performing a mathematical operation thatcalculates the π/2-phase component of the output select signal.
 10. TheDLL as in claim 9, wherein the phase detect signal further comprises upand down components, the up and down components determining the signalvalue of the feedback select and output select signals.
 11. The DLL asin claim 9, wherein the logic circuit comprises intercoupled adders. 12.The DLL as in claim 9, wherein the zero-phase component of the outputselect signal determines, via the second multiplexer, the first outputsignal, and the π/2-phase component of the output select signaldetermines, via the second multiplexer, a second output signal.
 13. TheDLL as in claim 12, further comprising a first inverter coupled with thefirst output signal and a second inverter coupled with the second outputsignal, the first inverter producing a third output signal and thesecond inverter producing a fourth output signal.
 14. A method ofdelaying a signal with a Delay Locked Loop (DLL), the method comprising:providing a phase detect signal to a control module, wherein the phasedetect signal represents a measure of a phase difference between areference clock signal and a feedback clock signal; deriving, via thecontrol module, an offset select signal and an output select signal;providing the reference clock signal to a delay line, the delay lineproviding a plurality of signal taps, each signal tap providing adistinct delay time, the delay line, thereby, providing a plurality ofselectable reference signals; providing the feedback select signal to afirst multiplexer and the output select signal to a second multiplexer;with the first multiplexer, selecting, via the feedback select signal, afirst selected reference signal from the plurality of selectablereference signals, the first multiplexer outputting the first selectedreference signal as the feedback clock signal; and with the secondmultiplexer, selecting, via the output select signal, a second selectedreference signal from the plurality of selectable reference signals, thesecond multiplexer outputting the second selected reference signal as anoutput signal of the DLL.
 15. The method of claim 14, further comprisingproviding the reference clock signal and the feedback clock signal to aphase detector, the phase detector producing the phase detect signal.16. The method of claim 15, further comprising isolating the phasedetect, feedback select, and output select into at least two phasecomponents.
 17. The method of claim 16, further comprising performingmathematical calculations with a logic circuit coupled to receive the atleast two phase components of the first selected feedback signal, thelogic circuit outputting a phase component of the output select signal.18. The method of claim 14, further comprising coupling a delay selectsignal with the control module so as to program a desired phase delay ofthe output signal.
 19. A method of delaying a signal within a DelayLocked Loop (DLL), the method comprising: deriving first and secondfeedback signals from a reference signal, the first and second feedbacksignals having different phases; and coupling the first and secondfeedback signals with a logic circuit, the logic circuit producing anoutput signal based on mathematical operations performed with the firstand second feedback signals by the logic circuit, wherein a multiplexeris coupled to receive the output signal at a select input, themultiplexer selecting, via the output select signal, a delay of thereference signal.
 20. (canceled)
 21. The method as in claim 19, whereinthe first feedback signal is at the same phase of the reference signal,and the second feedback signal is lagging the first feedback signal by aphase of 2π.
 22. The method as in claim 21, wherein the output signal isderived by performing the mathematical operations represented by theformula${\phi( \frac{\pi}{2} )} = \frac{( {{\phi( {2\pi} )} + {3{\phi(0)}}} )}{4}$where $\phi( \frac{\pi}{2} )$ is the output signal, φ(2π) isthe second feedback signal, and φ(0) is the first feedback signal. 23.(canceled)